A Multiple Input Multiple Output (MIMO) system refers to a system that can enhance transmission and reception efficiency of data by using multiple transmitting antennae and multiple receiving antennae. The MIMO technology includes a spatial diversity scheme and a spatial multiplexing scheme. The spatial diversity scheme may increase transmission reliability or may widen a cell range through a diversity gain. Thus, the spatial diversity scheme is suitable for data transmission with respect to a user equipment moving at a high speed. And, by transmitting different data simultaneously, the spatial multiplexing scheme may increase the data transmission rate without increasing the system bandwidth.
In the MIMO system, each transmitting antenna has an independent data channel. The transmitting antenna may signify a virtual antenna or a physical antenna. A reception entity may estimate a channel with respect to each transmitting antenna of a transmission entity, thereby being capable of receiving data transmitted from each transmitting antenna. Channel estimation refers to a process of recovering a received signal by compensating for a distortion in a signal, which is caused by fading. Herein, fading refers to an effect wherein the strength of a signal is changed rapidly due to a multi path-time delay in a wireless communication system environment. In order to perform channel estimation, a reference signal commonly known to the transmission entity and the reception entity is required. A reference signal may also be simply referred to as an RS (Reference Signal) or a Pilot. Also, the reception entity may determine the channel information based upon a measurement of the received reference signal and may feedback the determined channel information to the transmission entity.
A downlink reference signal corresponds to a pilot signal for coherent demodulation of downlink channel, such as PDSCH (Physical Downlink Shared CHannel), PCFICH (Physical Control Format Indicator CHannel), PHICH (Physical Hybrid Indicator CHannel), PDCCH (Physical Downlink Control CHannel), and so on. The downlink reference signal may include a Common Reference Signal (CRS) commonly shared by all user equipments within a cell, and a Dedicated Reference Signal (DRS) specified only for a specific user equipment. The Common Reference Signal (CRS) may also be referred to as a cell-specific reference signal. And, the Dedicated Reference Signal (DRS) may also be referred to as a UE-specific reference signal.